This invention relates to power tools and, more particularly, to a new and improved control system and method to correct for torque overshoot in such tools.
One area of use of the present invention is in tightening threaded fasteners, although the principles of the present invention can be variously applied. In controlled power tool systems used in tightening threaded fasteners the amount of torque required to turn the fastener one degree is known as the torque rate. In a typical electrically operated system, the fastening tool contains a motor connected through a set of speed reducing, torque amplifying gears, to any of a number of output heads connecting the tool to the socket being used to tighten the threaded fastener. At some point in the gear train a torque transducer is located which generates electrical signals proportional to the torque being transmitted through that point in the gearing. The on/off run status of the tool motor is controlled by a microprocessor-based meter/control unit.
In such a power tool, an electrical signal proportional to torque is fed from the torque transducer in the tool to the control unit in which a torque target has been set. When that target torque is reached, the run status command signal being sent from the control unit to the tool is turned off. The tool stops, but not quickly enough to prevent some torque overshoot above the target torque set on the controller. The time required to sense the torque, process the information and remove the run signal, along with the inertia in the decelerating high-speed elements in the tool, cause the rotating components to continue to rotate beyond the point at which they were to have stopped. This excess rotation, when transmitted through the tool to the fastener, can potentially drive the final fastener torque well beyond the target torque set in the control unit.
On a high torque rate threaded joint, the torque can be driven beyond the upper limit of acceptability for the joint, causing the joint to be considered unacceptable. The capability of the tool to control torque on a combined series of threaded fasteners of widely differing torque rates is a measure of tool accuracy, with minimum acceptable specifications for particular users. A competitive advantage exists for manufacturers of tools with the ability to control torque on such a combined series of joint types. A further competitive advantage exists for manufacturers whose fastening systems accomplish this control with a minimum of user involvement and complexity.
One way to correct the torque overshoot problem is to partially slow the tool at some point in the tightening sequence earlier than that point at which the measured torque is equal to the target torque. This is generally done by employing a downshift feature where at a pre-defined pre-torque torque value, the speed is lowered to a specified downshift speed value. The lower speed proportionally reduces only the portion of the torque overshoot produced by the excess rotation of the tool motor due to the inertia in the decelerating high-speed elements in the tool.
One shortcoming of the foregoing approach arises from the fact that it is convenient for a customer to use fastening systems operated with a common control unit. The downshift feature described hereinabove would therefore be active for every tightening sequence, not just the ones that would benefit from it. The lower speed is maintained even on joints that require many revolutions to reach the target torque, which causes delays in production and excessive electrical tool heating, with no significant improvement in final torque accuracy. Furthermore, it is inconvenient and sometimes impossible for a customer to setup these control units differently for each tool and fastener combination.
Another consideration arising from the foregoing downshift approach is that the magnitude of the total gear ratio in the tool has a significant effect on the torque overshoot, given a common motor and control unit combination. Lower total gear ratios transmit more of the excess motor rotation to the fastener, while higher ratios transmit less, making lower ratio tools particularly vulnerable to torque overshoot. The setting of the downshift parameter values must therefore be a compromise if low and high ratio tools are to be operated with a common motor and control unit combination. If the speed is set to favor the low ratio tools, high ratio tools using this setup will run slower than necessary causing aforementioned delays in production and excessive electric tool heating, particularly on low torque rate joints.